Many contemporary robotic working tools, such as robotic lawnmowers, are designed to work in a work area defined by a boundary wire through which a control signal is transmitted. The control signal will give rise to a magnetic field emanating from the boundary wire. As the magnetic field will have one polarity on one side of the boundary wire and a reverser polarity on an opposite side of the boundary wire, a robotic work tool will be able to determine when it crosses the boundary by detecting a shift in the polarity of the magnetic field. Furthermore, by knowing the time occurrence of a control signal, such as in a synchronized system, the robotic work tool will be able to determine an expected polarity of the sensed magnetic field and based on this determine if it is inside or outside a working area without having to cross the boundary wire. This is important from a safety perspective as it prevents a robotic work tool from being started outside a work area. It is also important that an robotic work tool is able to determine whether the boundary wire is active or not, that is whether a control signal is being transmitted through the boundary wire or not. To do this the robotic work tool must be able to continuously (possibly periodically) sense the magnetic field generated by the control signal to be able to determine that the control signal is active. If the robotic work tool would be allowed to operate without sensing the control signal, the robotic work tool would not be able to determine a failure of the boundary system and could possibly escape and cause damages to the surroundings of the work area.
It is therefore important that the robotic work tool is able to sense the magnetic field caused by the control signal all through the work area.
However, the European Parliament and Council Directive 1999/5/EC, also referred to as the Radio and Telecommunications (R&TTE) directive, has stipulated limitations to the allowed energy emitted from a magnetic field which restricts the maximum strength of a magnetic field which in turn limits the maximum allowed work area using one boundary wire. One option to increase a work area is to use several boundary wires each designating a boundary wire, which increases the cost of setting up a work area and installing a robotic work tool system.
There is thus a need for a robotic work tool system that is able to operate safely within a large work area without violating the R&TTE directive.